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Jan. 5, 2010 - If some pollutant triggers autism, it's widespread and not confined to specific geographic areas, finds a study of California autism clusters.
"People seeking the cause of autism have been looking for a smoking gun, but it will be more like dispersed land mines," Irva Hertz-Picciotto, PhD, MPH, tells WebMD.
Hertz-Picciotto and colleagues at the University of California, Davis MIND Institute analyzed mothers' residences on more than 2.4 million birth records that included nearly 10,000 autism cases in the California Department of Developmental Services (DDS) database.
The idea was to look for areas where there might be clusters of autism. If the increased risk of autism in these areas could not be explained by factors known to increase autism diagnosis, the search would be on for something in the environment of those areas that might trigger autism.
Sure enough, Hertz-Picciotto and colleagues found 10 autism clusters -- with at least a 70% higher incidence of autism than in the surrounding area -- in eight regions of California.
But as it turned out, there was indeed a factor that very likely explained the clusters. Most of the clusters were in areas where women tend to have high educational attainment. Autism cases in all of the clusters were more likely to be reported from families with highly educated mothers, Hertz-Picciotto says.
"I don't think people living in these areas need to be concerned about where their homes are. Thinking about moving away from these areas would be wholly inappropriate," she says.
The California DDS does not go out and look for kids with autism. Parents have to go to the DDS and seek services. Better educated women are more likely to know about these services -- and are more likely to have access to doctors who can diagnose their child's autism.
"The implication is that parents with better awareness of autism symptoms, with better economic means, can better test for early warning signs and find the right kind of help for their children. And that can contribute to increased autism prevalence," Andy Shih, PhD, vice president of scientific affairs for the advocacy group Autism Speaks, tells WebMD. Shih was not involved in the California study.
On the other hand, "it is entirely possible that people with higher education have some kind of exposure that increases autism risk," Hertz-Picciotto says.
But that's unlikely, she suggests. There clearly is a genetic predisposition to autism. But genes don't explain why some people get autism and others don't. Many researchers feel there has to be some kind of environmental trigger -- perhaps something a woman encounters during pregnancy or that a child encounters in infancy -- that triggers the disorder in susceptible children.
"Our study tells us probably the environmental causes of autism are not going to be found in local contamination, at least in California," Hertz-Picciotto says. "Whatever the environmental contributors are, they are probably more widespread and not linked to a hazardous local factor."